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Antarctic Drill: The Science of Ice

Oct 1, 2009

Geologist Trevor Williams, of Columbia University's Lamont-Doherty Earth Observatory, will be spending six weeks at Antarctica's McMurdo Station, taking part in a study of the sea-floor sediments beneath the Ross Ice Shelf. The results could aid our understanding of climate change. Williams will be filing reports here throughout the project.

ROSS ISLAND, Antarctica, Dec. 8 — For the first half of the Antarctic field season, the 10-foot-thick sea ice right next to McMurdo Station makes an excellent airfield. But with the warmer days of summer, surface melt makes landing planes trickier, and in some years the sea ice breaks up entirely. The solution: Move the airfield up to the ice shelf. It's less conveniently situated, but the ice there is much more stable.

In Antarctica, there are many different types of ice. The build-up and melt of the sea ice is one of the largest annual changes visible from space.

Ice shelves, on the other hand, are much longer-lived. They are fed by the glaciers and ice streams that drain the continent, pushing slowly seaward, and they eventually calve off as icebergs. ANDRILL is investigating the history of the 600-mile-wide Ross Ice Shelf, which is three-quarters the size of Texas (or, in the metric system, about the size of France). It gets as thick as 3000 feet thick in some areas, and melts slowly from the base, becoming gradually thinner by the time it reaches the sea (it's about 250 feet thick at the drill site). The East and West Antarctic ice sheets are bigger still, reaching two and a half miles thick and containing 70% of the world's freshwater.

(Years ago on the Ocean Drilling Program research ship on the other side of Antarctica, I was chatting with a Texan crew member as we watched an approaching iceberg. "That's one impressive iceberg," I said. "Sure is," my friend replied. "But they're bigger in Texas!")

Why is the Ross Ice Shelf important? Like the ice in your soda, or Archimedes in his bathtub, the floating ice shelf has displaced its weight in seawater. If it melted, it wouldn't affect sea level. However, as Penn State glaciologist Richard Alley points out, ice shelves act as buttresses to the continental ice streams. If the Ross Ice Shelf collapsed, the ice streams flowing out from West Antarctica would speed up and that would cause sea level to rise—something we are keen to avoid.

The ANDRILL objective for this year is to look at the sediments trapped under the modern day ice shelf (see graphic at bottom right) in an effort to model how much—and how rapidly—the Ross Ice Shelf has changed. Layers of sediment that date to times when the site was covered by ice are coarse grained and include large pieces of gravel (the geological term is "diamict"). Sediment from the years when the drill site was covered by open ocean are made of diatoms, tiny marine plankton ("diatomite"). These very different rock types give geologists a clear picture of what conditions were like in the geological past. We are drilling back in time: The deeper we drill, the older the sediments get.

We are finding more than just gravel and plankton: Ash and pumice that erupted from Mount Erebus and other local volcanoes is trapped in the seabed. We can use the potassium-bearing volcanic minerals to give ages to the sediment sequence—very useful. And the faults and fractures in the cores give clues to the geological tectonic history of Antarctica.

But enough with the science—there's been a small disaster at the bottom of the world. Coming home from the drill site, I stopped to take photos of the ice pressure ridges (see top right). As I stepped back into the pickup truck, I put my camera on my lap. It was only when I got out at McMurdo that I realized the camera had fallen to the ground. This would have been fine had the pickup truck not subsequently reversed over it.

So far, drilling has reached an impressive 740 meters below the sea floor. But still no penguins! —Trevor Williams

You are here: McMurdo Station on a satellite image of Antarctica. COURTESY OF USGS (CLICK TO ENLARGE)

The annual sea ice breaks up quickly at this time of year. COURTESY MODIS RAPID RESPONSE PROJECT AT NASA/GSFC (CLICK TO ENLARGE, VISIT http://rapidfire.sci.gsfc.nasa.gov/subsets/?RossSea/ FOR DAILY IMAGE UPDATES)

The sediments tell the story. An example of rock types and interpretation from the ANDRILL core. COURTESY OF RICHARD LEVY/ANDRILL. (CLICK TO ENLARGE)

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